During the operation of industrial circulating cooling water systems, water evaporation and wind loss cause the circulating water to concentrate, containing excessive salts, increasing anions and cations, and changing pH values, leading to deterioration of water quality. At the same time, microorganisms in the circulating water multiply rapidly and are suitable for a growing environment. And scaling control, corrosion control, and microbial control also require circulating water treatment.
(1) Scale is formed by the continuous evaporation of circulating water during the cooling process, resulting in an increasing salt concentration in the water that exceeds the solubility of certain salts and precipitates. Common scales include calcium carbonate, calcium phosphate, magnesium silicate, etc. These scales have a dense texture and can reduce heat transfer efficiency.
(2) Dirt is mainly composed of organic matter, microbial colonies and secretions, sediment, dust, etc. in water. These scales have a soft texture, which can reduce heat transfer efficiency and cause corrosion under the scale.
(3) The corrosion of circulating water heat exchange equipment is mainly electrochemical corrosion. Equipment manufacturing defects, sufficient oxygen in water, corrosive ions in water (such as Cl -, Fe2+, Cu2+), and dirt generated by microbial secretions can all accelerate corrosion.
(4) Microbial sludge proliferates extensively in circulating water, leading to water quality deterioration, odor, blackening, and the deposition or even blockage of a large amount of fouling in cooling towers. This greatly reduces the cooling and heat dissipation effect, and exacerbates equipment corrosion.
microorganism
The cooling water system firstly introduces a large amount of air into the cooling tower during the evaporation process of water, and microorganisms also enter the cooling water; Secondly, the supplementary water of the cooling water system contains a certain amount of microorganisms, which will also enter the cooling water system.
Algae can undergo photosynthesis with carbon sources such as carbon dioxide and bicarbonate in water under sunlight, absorbing carbon and releasing oxygen. Therefore, when algae proliferate in large numbers, it increases the dissolved oxygen content in the water, promotes oxygen depolarization, and accelerates the corrosion process. At the same time, the formed sludge will reduce the cooling efficiency of the cooling tower, leading to the deterioration and decay of the wood.
The adhesion of clay to metal surfaces can cause severe corrosion under scale, while also hindering the protective effect of corrosion and scale inhibitors on metals, making it impossible for the agents to achieve their intended corrosion and scale inhibition effects. These issues can lead to the inability of the cooling water system to operate safely for a long time, seriously affecting production and causing significant economic losses. Therefore, the harm of microorganisms is equally serious as the impact of scale and corrosion on cooling water systems, and even compared to the three, controlling the harm of microorganisms should be given priority.
The movement of microorganisms in circulating water can be analyzed and determined by the following chemical methods:
Residual chlorine: Adding chlorine to eliminate bacteria, but excessive residual chlorine indicates severe bacterial growth, which greatly increases the consumption of chlorine in circulating water.
Ammonia: Under normal circumstances, circulating water does not contain ammonia, but leakage of process media or inhalation of ammonia into the air can cause ammonia content in the water. Therefore, it is necessary to be vigilant and actively search for the source of ammonia leakage, while also paying attention to whether the water contains nitrite ions. It is best to control the ammonia content at 1
NO2-: When ammonia and nitrite ions appear in water, it indicates that nitrite bacteria have already converted ammonia into nitrite ions in the water.
Chemical Oxygen Demand: Severe microbial growth in water can increase COD content, as the mucus secreted by bacteria increases the organic matter content in the water. By analyzing chemical oxygen demand, the trend of microbial changes in water can be observed. Under normal circumstances, it is best for COD in water to be less than 5mg/L (KMnO4 method).
The harm caused by microorganisms in circulating water is very serious. If measures are taken after the harm occurs, it often results in twice the effort and consumes a large amount of killing agents and money. Therefore, it is crucial to comprehensively monitor the microbial situation of circulating cooling water in advance.
Concentration ratio of circulating water
It is an important comprehensive indicator for measuring the quality of water control, based on the comparison of supplementary water. However, if the concentration factor is too high, the tendency of water scaling will increase, and the difficulty of scaling control and corrosion control will also increase. Water treatment agents may become ineffective, which is not conducive to microbial control. Therefore, there needs to be a reasonable control index for the concentration ratio of circulating water.
Formation of scale
In the circulating water system, scale is formed by supersaturated water-soluble components. Various salts such as bicarbonate, carbonate, chloride, silicate, etc. are dissolved in water, among which dissolved bicarbonate such as Ca (HCO3) 2 and Mg (HCO3) 2 are the most unstable and easily decompose to form carbonates.
Therefore, when there is a large amount of dissolved bicarbonate in the cooling water, the water flow passing through the surface of the heat exchanger, especially the surface with higher temperature, will undergo thermal decomposition; When phosphate and calcium ions are dissolved in water, precipitation of calcium phosphate will also occur; Calcium carbonate and Ca3 (PO4) 2 are both difficult to dissolve salts, different from general salts. Their solubility does not increase with increasing temperature, but decreases with increasing temperature.
Therefore, on the heat transfer surface of the heat exchanger, these insoluble salts are easily supersaturated and crystallize in water, especially when the water flow rate is low or the heat transfer surface is rough. These crystalline precipitates will deposit on the heat transfer surface, forming what is commonly known as scale. Due to the dense and hard crystallization of these scales, they are also known as hard scales. Common components of scale include calcium carbonate, calcium sulfate, calcium phosphate, magnesium salts, and silicates.
Based on the characteristics and process conditions of the enterprise's circulating water system, combined with the local water quality situation, select a suitable water treatment plan for the enterprise. By taking measures such as adding chemicals, the circulating water indicators are controlled within the specified range, ensuring the operation cycle of production equipment and improving the utilization rate of circulating water. The application of circulating water treatment technology not only brings economic benefits to enterprises, but also generates good social benefits. Therefore, the application of circulating water treatment technology is essential.
